use Comparable correctly

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/history/TagEntry.java

@@ -31,7 +31,7 @@
  *
  * @author Stanislav Kozina
  */
-public abstract class TagEntry implements Comparable {
+public abstract class TagEntry implements Comparable<TagEntry> {
 
     protected int revision;
     /**
@@ -92,13 +92,11 @@ public Date getDate() {
      * @return 1 for greater, 0 for equal and -1 for smaller objects.
      */
     @Override
-    public int compareTo(Object aThat) {
-        if (this == aThat) {
+    public int compareTo(TagEntry that) {
+        if (this == that) {
             return 0;
         }
 
-        final TagEntry that = (TagEntry) aThat;
-
         if (this.revision != NOREV) {
             return ((Integer) this.revision).compareTo(that.revision);
         }